The present invention relates to valve control systems for internal combustion engine poppet valves, and more particularly, to such valve control systems which are capable of controlling the amount of the valve lift, the timing of the valve lift, and the duration of the valve event (valve lift).
As is well know to those skilled in the internal combustion engine art, conventional camshaft and rocker arm type valve gear trains are relatively simple and have been generally effective in commercial use. However, the conventional camshaft actuated valve gear train has typically represented a compromise in regard to engine performance. At relatively low speeds, the engine poppet valves open more than is needed (thus resulting in pumping energy losses), while at relatively higher engine speeds, the valves do not open enough to get the flow quantity of air-fuel mixture necessary to achieve optimum engine performance.
In addition, it is now understood that engine efficiency can be improved by varying the timing of the opening and closing of the poppet valves as a function of engine speed, and also as a function of load on the engine. One known method of varying the timing of the opening and closing of the engine poppet valves is by means of a variable cam phase change device (xe2x80x9cvariable cam phaserxe2x80x9d. The function of such a cam phaser device is to vary the angular position of the camshaft, relative to the angular position of the crankshaft.
Those skilled in the valve gear train art have, for many years, been developing various systems for variable valve actuation/variable valve timing (VVA/VVT) for modifying the amount of valve lift and/or the timing of the valve lift in valve gear trains of the type driven by a camshaft. More recently, the assignee of the present invention has developed a VVT system which is able to vary both the amount of lift and the timing of the lift simultaneously in a valve gear train having a camshaft as its input, this VVT system being illustrated and described in co-pending application U.S. Ser. No. 09/841,572, filed Apr. 24, 2001 in the name of Majo Cecur for a xe2x80x9cRocker Arm Device For Simultaneous Control Of Valve Lift And Relative Timing In A Combustion Enginexe2x80x9d, also assigned to the assignee of the present invention, and incorporated herein by reference.
Although the VVT system of the co-pending application appears to be a substantial improvement over the known prior art, and is able to adjust timing as much as is desired, the referenced system still has limitations whereby neither the valve lift nor the duration of the valve opening can ever go to zero.
In an effort to achieve a valve control system which is truly capable of fully variable valve train (FVVT) operation, those skilled in the art have developed various camless valve control systems in which the engine poppet valve has its opening and closing controlled directly by a valve actuator which may be electromagnetic or electro-hydraulic. Although such camless systems have certain benefits, such as infinitely variable valve lift, duration, and timing, and eliminating the frictional losses inherent in camshaft and rocker arm type valve gear train, there are still a number of disadvantages to the camless systems. By way of example only, in a typical camless system, it is necessary to provide sensors capable of sensing the position of the engine poppet valve and providing a feedback signal to control the operation of the electrical/electronic portion of the valve actuator. The sensors capable of performing the function just described are not yet commercially available, but even if they were, the requirement for such sensors would add substantially to the overall cost and complexity of the typical camless valve control system.
Another problem associated with the conventional camshaft and rocker arm type of valve gear trains is the need to provide xe2x80x9crampsxe2x80x9d to finely control the speed of the opening and closing of the engine poppet valve. Such ramps are typically provided by grinding an appropriate shape on the cam profile of the camshaft, at both the beginning and end of the lift profile. Unfortunately, the accurate grinding of a pair of such shapes for each cam lobe adds substantially to the overall manufacturing cost of the camshaft.
Accordingly, it is an object of the present invention to provide a valve control system for an engine poppet valve which is capable of achieving fully variable valve train operation.
It is a more specific object of the present invention to provide such a valve control system in which both the valve lift and the timing of the valve lift may be controlled simultaneously, but independently of each other.
It is an even more specific object of the present invention to provide a valve control system of the type which achieves the above-stated objects in which the valve lift may be varied anywhere between a maximum lift and a theoretical zero lift; the duration of the lift may be varied anywhere between a maximum duration and a theoretical zero duration; and the timing of the valve opening and closing may be varied as desired to achieve optimum engine performance.
The above and other objects of the invention are accomplished by the provision of a valve control system for an internal combustion engine of the type including a cylinder head, an engine poppet valve, and a valve actuating camshaft defining a cam profile. The valve control system comprises a rocker arm having a first portion fixed to pivot relative to the cylinder head and a second portion operable to transmit cyclic opening and closing motion to the engine poppet valve. A cam follower assembly includes a first cam follower member in engagement with the cam profile whereby cyclic rise and fall motion is transmitted from the cam profile to the cam follower assembly in response to rotation of the camshaft.
The improved valve control system is characterized by a cam member disposed for to and fro movement relative to the camshaft and defining a cam surface disposed in a generally face-to-face relationship to an engagement surface defined by the rocker arm. The cam follower assembly includes a second cam follower member in operable engagement with both the engagement surface defined by the rocker arm and the cam surface defined by the cam member, and being disposed therebetween. The cam follower assembly further includes a rigid follower linkage interconnecting the first and second cam follower members, thus transmitting the cyclic rise and fall motion to the second cam follower member. An arrangement includes a link member having one end fixed relative to the cam member and the other end operably associated with an input member to vary the to and fro position of the cam member. An adjustable timing linkage has one end fixed to the first cam follower member and is operable, in response to a timing input motion, to advance or retard the point of contact of the first cam follower member on the cam profile.